The Dallas Cowboys’ best linebacker walked through the locker room Monday afternoon.

Sean Lee won’t play this season, though, having torn the anterior cruciate ligament in his left knee in off-season workouts.

His absence on the field leaves a huge hole in the team’s defense.

The Cowboys have spent the entire preseason trying to replace Lee, who finished second on the team in tackles last season with 123 despite missing five games.

“We feel good about the progress that they’ve made, and the versatility of those guys, how each of those guys has stepped up,” Cowboys coach Jason Garrett said Monday. “Sean Lee got hurt in the spring, and initially there was a void there. The question we had was: Who’s going to step up into that void?

“These guys have done a lot of work over the course of OTAs, minicamp and training camp and preseason games to show that they can step up into that. We probably won’t do it the same way we’ve done it in the past because of Sean’s presence, but we feel like with that group in there, we’re going to get all the jobs that we need to get accomplished, and each of those guys is going to have a role for us.”

After experimenting with their linebacker group, the Cowboys appear ready to trust the twice-retired Rolando McClain as their starting middle linebacker, with Bruce Carter moving from weakside to strongside and Justin Durant to the weakside.

The linebackers’ play will go a long way in determining if the defense can improve from last season when it ranked 32nd in the NFL and gave up the third-most yards in NFL history.

Like the Cowboys’ defense, McClain has a lot to prove.

“We’ve just got to get it back to the Dallas Cowboys of old,” McClain said. “I know some guys or this defense hasn’t met the standards of the Dallas Cowboys of late, but every day is a new day, every year is a new year, every snap is a new snap. We can’t dwell on the past. All we can do now is get better. We like the work we’ve done in practice, in training camp. We’ve been busting our butt.”

McClain, 25, is a former eighth overall pick of the Oakland Raiders who spent last season out of football after twice retiring. Since joining the Cowboys this summer, McClain has shown flashes of the potential that made him an All-American at Alabama.

Sorely in need of defensive playmakers, the Cowboys hope they have hit on McClain.

“I think he’s made a lot of progress,” Garrett said. “Whenever you’re a new player coming into a system, you need to just play in the system. You need to get reps. That’s particularly the case with a position like linebacker where there’s a lot to do in terms of communication and understanding what the whole defense is doing.

“He has a lot of those traits. Coming in, it was a matter of learning what we were asking him to do. He embraced that, and I think he’s going to be a part of that mix at linebacker. He’s demonstrated both in practice and in preseason games that he’s worthy of some kind of an opportunity.”

McClain has had to prove his commitment ever since the Cowboys traded with the Ravens for his rights. He missed several practices with various injuries in training camp, raising questions, but McClain insists he loves the game and is in a better place than he was his last season in the NFL.

“When the ball is snapped, the lights are on, either somebody’s going to hit you or you’re going to hit somebody, so you better be ready,” McClain said. “I still love the game like I’m an 8-, 9-year-old kid. It’s just about getting back into it, building chemistry with some of these guys, some trust and just playing and having fun really.”